Because your are lumping them all into one group. Treat them as an individual, watch and listen they will actually tell you everything you need to know. You just have to pay attention. Ok MOST will. There are those select few that like to play games. Just skip over them.
Girls are complicated because they are so interesting and creative. Guys are thereby permanently kept guessing over what do do next - and that is Mother Nature's protective mechanism for young girls. Any guy that reckons he understands women is self-deluding and is in for a rude shock!!!
Girls and boys, or men and women, see life and the world in different frames of mind. They always have and they always will. It's the balance between the points of view that give you a true view of things. But this means that beyond a certain point there are things about the females of the species that the males will never understand. And to an equal degree there are things about the male of the species that the females will never understand. Hence the confusion. The true problem arrives when either side insists that ITS point of view is correct and the other is wrong; that one is superior and the other inferior. Superior/inferior doesn't enter into it; they're just different. When you don't understand the other gender, just accept that you don't understand and don't worry about it.